Acquired immunodeficiency syndrome (AIDS) is caused by a retrovirus identified as the human immunodeficiency virus (HIV). There has been intense effort to develop a vaccine. These efforts have focused on inducing antibodies to the HIV envelope protein. Recent efforts have used subunit vaccines where an HIV protein, rather than attenuated or killed virus, is used as the immunogen in the vaccine for safety reasons. Subunit vaccines generally include gp120, the portion of the HIV envelope protein which is on the surface of the virus.
The HIV envelope protein has been extensively described, and the amino acid and RNA sequences encoding HIV envelope from a number of HIV strains are known (Myers, G. et al., 1992. Human Retroviruses and AIDS. A compilation and analysis of nucleic acid and amino acid sequences. Los Alamos National Laboratory, Los Alamos, N. Mex.). The HIV envelope protein is a glycoprotein of about 160 kd (gp160) which is anchored in the membrane bilayer at its carboxyl terminal region. The N-terminal segment, gp120, protrudes into the aqueous environment surrounding the virion and the C-terminal segment, gp41, spans the membrane. Via a host-cell mediated process, gp160 is cleaved to form gp120 and the integral membrane protein gp41. As there is no covalent attachment between gp120 and gp41, free gp120 is released from the surface of virions and infected cells.
The gp120 molecule consists of a polypeptide core of 60,000 daltons which is extensively modified by N-linked glycosylation to increase the apparent molecular weight of the molecule to 120,000 daltons. The amino acid sequence of gp120 contains five relatively conserved domains interspersed with five hypervariable domains. The positions of the 18 cysteine residues in the gp120 primary sequence, and the positions of 13 of the approximately 24 N-linked glycosylation sites in the gp120 sequence are common to all gp120 sequences. The hypervariable domains contain extensive amino acid substitutions, insertions and deletions. Sequence variations in these domains result in up to 30% overall sequence variability between gp120 molecules from the various viral isolates. Despite this variation, all gp120 sequences preserve the virus's ability to bind to the viral receptor CD4 and to interact with gp41 to induce fusion of the viral and host cell membranes.
Gp120 has been the object of intensive investigation as a vaccine candidate for subunit vaccines, as the viral protein which is most likely to be accessible to immune attack. Gp120 is considered to be a good candidate for a subunit vaccine, because (i) gp120 is known to possess the CD4 binding domain by which HIV attaches to its target cells, (ii) HIV infectivity can be neutralized in vitro by antibodies to gp 120, (iii) the majority of the in vitro neutralizing activity present in the serum of HIV infected individuals can be removed with a gp120 affinity column, and (iv) the gp120/gp41 complex appears to be essential for the transmission of HIV by cell-to-cell fusion.
The identification of epitopes recognized by virus neutralizing antibodies is critical for the rational design of vaccines effective against HIV-1 infection. One way in which antibodies would be expected to neutralize HIV-1 infection is by blocking the binding of the HIV-1 envelope glycoprotein, gp120, to its cellular receptor, CD4. However, it has been surprising that the CD4 blocking activity, readily demonstrated in sera from HIV-1 infected individuals (31, 44) and animals immunized with recombinant envelope glycoproteins (1-3), has not always correlated with neutralizing activity (2, 31, 44). Results obtained with monoclonal antibodies have shown that while some of the monoclonal antibodies that block the binding of gp120 to CD4 possess neutralizing activity, others do not (4, 7, 16, 26, 33, 35, 43, 45). When the neutralizing activity of CD4 blocking monoclonal antibodies are compared to those directed to the principal neutralizing determinant (PND) located in the third variable domain (V3 domain) of gp120 (10, 39), the CD4 blocking antibodies appear to be significantly less potent. Thus, CD4 blocking monoclonal antibodies typically exhibit 50% inhibitory concentration values (IC.sub.50) in the 1-10 .mu.g/ml range (4, 16, 26, 33, 35, 43, 45) whereas PND directed monoclonal antibodies typically exhibit IC.sub.50 values in the 0.1 to 1.0 .mu.g/ml range (23, 33, 42).
Subunit vaccines, based on gp120 or another viral protein, that can effectively induce antibodies that neutralize HIV are still being sought. However, to date no vaccine has not been effective in conferring protection against HIV infection.